In recent years, as a reinforcing method for an existing or new steel structure, a continuous fiber sheet bonding method such as a carbon-fiber sheet bonding method or an aramid fiber sheet bonding method that attaches or winds a continuous reinforcing-fiber sheet such as a carbon fiber sheet or an aramid fiber sheet to the surface of the steel structure is given. A method in which a fiber sheet obtained by impregnating an unhardened matrix resin in continuous fiber bundles is bonded to adhere to a steel structure and then hardened is also given.
Furthermore, in order to omit the impregnation of a resin on site, an FRP plate bonding-reinforcing method in which a factory-produced FRP plate having a thickness of 1 to 2 mm and a width of about 5 cm is bonded to a steel structure by using a putty-like adhesive resin is also developed.
A steel structure reinforced by the above methods can obtain an enhanced reinforcing effect by a fiber sheet as long as the fiber sheet is integrally bonded to the steel structure. However, when the steel structure is deformed with a load, a desired object cannot be achieved when the fiber sheet is peeled away from the steel structure surface before the fiber sheet is broken.
Thus, Patent Document 1 discloses a method in which a buffer material layer is formed on the surface of a steel structure and, thereafter, a fiber sheet is bonded thereto with an adhesive agent to reinforce the steel structure. It is disclosed that, as the buffer material layer, a resin such as a thermosetting resin or a thermoplastic resin can be used. In addition, it is disclosed that a tensile elasticity modulus at 23° C. when the resin is singularly hardened is 0.1 to 50 N/mm2.